Hydrocarbon synthesis from hydrogen and carbon monoxide in the presence of a Fischer-Tropsch catalyst is commonly known as Fischer-Tropsch (FT) synthesis. FT synthesis forms part of gas-to-liquids, coal-to-liquids, and biomass-to-liquids processes in which natural gas, coal, and biomass respectively are usually converted by means of a three step process into liquid hydrocarbons. The three process steps are normally (i) production of synthesis gas (or ‘syngas’) comprising a mixture of hydrogen and carbon monoxide from natural gas, coal, or biomass respectively, (ii) conversion of the syngas into a waxy hydrocarbons or syncrude by means of FT synthesis, and (iii) a hydrocracking or hydrotreating step to convert the waxy syncrude into liquid transportation fuels such as diesel, petrol, jet fuel, as well as naphtha.
During the FT synthesis described in step (ii) above the syngas in the form of CO and H2 is contacted with a FT synthesis catalyst under FT synthesis conditions to produce the waxy hydrocarbons. One type of catalyst which is often used in low temperature FT (LTFT) synthesis comprises an active catalyst component such as Co on a catalyst support such as alumina, silica, titania, magnesia or the like.
Contamination of the waxy hydrocarbon product produced during FT synthesis with ultra fine particulate matter derived from the support such as alumina, and the active catalyst component such as Co, was experienced. This resulted in loss of the expensive active catalyst component as well as fouling of the downstream processes described in (iii) above with the support and active catalyst component ultra fine particles. It is believed that this wax product contamination is as a result of one or both of: (a) Catalyst support dissolution during aqueous impregnation of the catalyst support with the active catalyst component (during preparation of the catalyst) which may result in precipitation and coating of the bulk support material with a physically bonded amorphous layer of the support material whereon the active catalyst component is deposited. This amorphous layer is insufficiently anchored and results in dislodgement of and washing out of active catalyst component rich ultra fine particles during FT synthesis; and (b) The FT synthesis catalyst is susceptible to hydrothermal attack that is inherent to realistic FT synthesis conditions. Such a hydrothermal attack on exposed and unprotected support material will result in contamination of the waxy hydrocarbon product with ultra fine particulate matter rich in the active catalyst component.
WO 99/42214, WO 02/07883, WO 03/012008 and U.S. Pat. No. 7,365,040 all disclose modification of a FT synthesis catalyst support with a modifying component to reduce the dissolution of the catalyst support in aqueous environment, including hydrothermal attack, thereby to reduce the negative effect of ultra fine particles rich in active catalyst component contaminating the waxy hydrocarbon product.
WO 99/42214, WO 02/07883, and U.S. Pat. No. 7,365,040 all disclose modification of a FT synthesis catalyst support by impregnation of the support with the modifying component carried in an organic solvent such as ethanol. Water is specifically avoided in order to avoid dissolution of the support in an aqueous environment during the support modification process.
WO 2009/049280 discloses modification of a catalyst support by impregnating the support with a modifying component carried in water. WO 2009/049280 is not limited to the preparation of FT catalysts and accordingly the problem associated with support dissolution in an aqueous medium does not play such an important role in that case. It should be noted that WO 2009/049280 discloses, on page 17, that when the use of water is compared to the use of anhydrous ethanol during impregnation of the support with the modifying component, a lower silicon content on the support is achieved when water is used. This is accordingly a disadvantage associated with water as an impregnating liquid medium.
Most surprisingly, it has now been found that when a certain mixture of water and an organic solvent was used to impregnate a modifying component onto a catalyst support, it may result in a higher modifying component content being deposited on the support compared to when no water is used during impregnation. It thus resulted in better utilisation of the modifying component. This is contrary to what is expected from the teachings of WO 2009/049280 set out above, namely that the use of water instead of ethanol as an impregnating liquid medium resulted in lower usage of the modifying component. This higher usage of the modifying component resulted in a higher loading of the modifying component or alternatively less wastage of the modifying component. It is well known that a higher loading of the modifying component results in a lower solubility of the catalyst support in water. Surprisingly, when specified amounts of water were used in the impregnating liquid medium, it resulted in improved attrition resistance of the modified catalyst support compared to the use of only water (and in some cases of only ethanol) as the impregnating liquid medium.